1. Field of the Invention
The present invention relates to image display apparatus and, more particularly, to image display apparatus suitably applicable to head-mounted displays or the like adapted to enable observation of an enlarged view of image information displayed on a reflective liquid-crystal display element as a display element through an optical element having properly set free-form surfaces.
2. Related Background Art
There are conventionally well-known image display apparatus of a head-mounted type (head-mounted displays) using the display element of a CRT, an LCD, or the like and constructed to enlarge an image displayed on such a display element through an optical system to permit observation of an enlarged view of the displayed image.
FIG. 10 is a schematic diagram of main part of a conventional image display device using a coaxial concave reflector. In the same figure light from the image displayed on the display element 61 is reflected by a half mirror 62 to be incident to the concave mirror 63. The light reflected by the concave mirror 63 is guided through the half mirror 62 to an observer E. The image displayed on the display element 61 is focused as an enlarged virtual image by the concave mirror 63. This permits the observer to observe the enlarged virtual image of the image displayed on the display element 61.
For example, Japanese Patent Applications Laid-Open No. 07-333551, No. 08-50256, No. 08-160340, No. 08-179238, etc. describe proposals of the image display devices reduced in the thickness of the entire apparatus by using an LCD (liquid-crystal display) as a display means for displaying an image and a slim prism as an observation optical system.
FIG. 11 is a schematic diagram of main part of the image display device proposed in Japanese Patent Application Laid-Open No. 07-333551. In FIG. 11, the light emitted from the LCD 51 is made incident to an entrance surface 53 of the compact prism 52. Then the light is folded between a total reflection surface 54 with curvature and a reflective surface 55 in the compact prism 52 and thereafter is guided out of the compact prism 52 through the surface 54 toward the observer E. This results in forming a virtual image of the image displayed on the display means (LCD) 51 so as to permit the observer E to observe the virtual image. The reflective surface 55 of the compact prism 52 is a decentered free-form surface comprised of a decentered rotationally asymmetric surface (a surface with optical powers differing depending upon azimuths, which is so called a free-form surface).
The type of the optical system illustrated in FIG. 11 has such a feature that it is easier to decrease the thickness of the entire apparatus and increase the angle of view of the observation field than in the case of the type using the conventional coaxial concave mirror illustrated in FIG. 10.
With the conventional image display devices such as the head-mounted displays, goggle-type displays, etc., there are particular demands for further decrease in the size and weight of the entire apparatus, because these devices are mounted on the head. Particularly, they are preferably constructed to be thin in the direction of the visual axis of the observer in view of the weight balance, the appearance, and so on. Another important subject is to widen the angle of observation view in order to give strong appeal to the observation of the image displayed on the display means.
In the image display device such as the thin head-mounted display (HMD) of the type illustrated in FIG. 11, if the display element is constructed of a reflective display element with a high aperture ratio advantageous in compactification, an illumination system 70 for illuminating the display element 74 will be needed, for example, between the reflective display element 74 and the entrance surface 53 of the compact prism 52, as illustrated in FIG. 12.
The illumination system 70 herein is composed, for example, of a light source 71, a condenser lens 72 for condensing diverging light from the light source 71, a prism 73 including a half mirror surface 73a which reflects the light from the condenser lens 72 to illuminate the display element 74 and which makes the angle of 45xc2x0 with the display surface of the display element 74, and so on. In the image display apparatus, the use of the reflective display element raises the necessity for the placement of the illumination system for illuminating it between the display element 74 and the compact prism 52 and, in turn, the need for increasing the space between the prism 52 and the display element 74, as illustrated in FIG. 12, thus posing the problem of decrease in the angle of view or the like with failing to take advantage of the feature of this type.
An object of the present invention is to provide image display apparatus that can accomplish both compactness of apparatus and a wide angle of view by properly setting construction of the illumination optical system for illuminating the display means, a display optical system for guiding the light from the display means to the observer""s eyes, for example, an optical means consisting of a prism body with refracting action, etc. for observation of the image information displayed on the reflective display means of the liquid-crystal display or the like in the image display apparatus of HMD or the like and that can realize observation of the image with excellent image quality without bringing about degradation of contrast, unevenness of contrast in the screen, etc. in the observation field.
An image display apparatus according to one aspect of the present invention is an image display apparatus for permitting an observer to observe image information displayed on display means, comprising: reflective display means; an illumination optical system having a reflective surface A for guiding light from light source means to the display means; and a display optical system for guiding light from the display means to an observation position of the observer, wherein when principal rays are defined as rays emerging from a display surface of the display means and impinging upon the center of an exit pupil and when a center-view-angle principal ray is defined as a ray emerging from the center of the display surface of the display means and passing the center of the exit pupil of the display optical system, the display optical system comprises a decentered reflective surface or a plurality of decentered optical systems decentered relative to the center-view-angle principal ray and having a power, wherein when illumination principal rays are defined as rays emerging from the light source means and guided onto the display surface by the illumination optical system to become the principal rays, an angle of incidence of each illumination principal ray onto the display surface and an angle of emergence of a corresponding principal ray from the display surface make a predetermined angle, and wherein when a center illumination principal ray is defined as a ray emerging from the light source means and guided to the center of the display surface by the illumination optical system to become the center-view-angle principal ray, when a coordinate system is so defined that a yz plane is set on a surface formed by incidence and emergence of the center illumination principal ray to and from the reflective surface A and that a z-axis is taken along a direction of a normal to the reflective surface A in the yz plane, a y-axis along a tangent direction thereto, and an x-axis along a direction normal to the yz plane, and when the illumination principal rays are projected onto the xz plane of the coordinate system, the illumination principal rays projected onto the xz plane are incident at respective angles of incidence approximately equal to each other to the reflective surface A.
In a further aspect of the present invention, said reflective surface A acts to totally reflect the light when the light from said light source means is guided to said display means and said reflective surface A acts to transmit the light when the light from the display means is guided to the observation position.
In a further aspect of the present invention, the apparatus is constructed so that in said coordinate system, when said illumination principal rays are projected onto the yz plane, the illumination principal rays projected onto the yz plane are incident at respective angles of incidence approximately equal to each other to said reflective surface A.
In a further aspect of the present invention, an optical member having said reflective surface A comprises at least one curved surface.
In a further aspect of the present invention, said reflective surface A is a surface having no power at least in the direction of the x-axis.
In a further aspect of the present invention, said illumination optical system comprises a first polarizer for converting the light from said light source means into linearly polarized light.
In a further aspect of the present invention, said display optical system comprises a second polarizer whose transmission axis is approximately perpendicular to that of said first polarizer.
In a further aspect of the present invention, said illumination optical system comprises an optical element having a positive power and light transmitted by the optical element is made incident to said first polarizer.
In a further aspect of the present invention, said optical element having the positive power comprises a reflective surface.
An image display apparatus according to another aspect of the present invention is an image display apparatus for permitting an observer to observe image information displayed on display means, comprising: reflective display means; an illumination optical system having a reflective surface A for guiding light from light source means to the display means; and a display optical system for guiding light from the display means to an observation position of the observer, wherein when principal rays are defined as rays emerging from a display surface of the display means and impinging upon the center of an exit pupil and when a center-view-angle principal ray is defined as a ray emerging from the center of the display surface of the display means and passing the center of the exit pupil of the display optical system, the display optical system comprises a decentered reflective surface or a plurality of decentered optical systems decentered relative to the center-view-angle principal ray and having a power, wherein when illumination principal rays are defined as rays emerging from the light source means and guided onto the display surface by the illumination optical system to become the principal rays and when xcex2 represents an angle between the reflective surface A and the display surface of the display means, the angle xcex2 satisfies the relation of xcex2 less than 45xc2x0, and wherein when a center illumination principal ray is defined as a ray emerging from the light source means and guided to the center of the display surface by the illumination optical system to become the center-view-angle principal ray, when a coordinate system is so defined that a yz plane is set on a surface formed by incidence and emergence of the center illumination principal ray to and from the reflective surface A and that a z-axis is taken along a direction of a normal to the reflective surface A in the yz plane, a y-axis along a tangent direction thereto, and an x-axis along a direction normal to the yz plane, and when the illumination principal rays are projected onto the xz plane of the coordinate system, the illumination principal rays projected onto the xz plane are incident at respective angles of incidence approximately equal to each other to the reflective surface A.
In the above-stated image display apparatus according to a further aspect of the present invention, at least one surface out of said plurality of decentered reflective surfaces is a rotationally asymmetric surface.
In the above-stated image display apparatus according to a further aspect of the present invention, said display optical system comprises an optical element integrally formed of said plurality of decentered reflective surfaces, an entrance surface, and an exit surface.
In the above-stated image display apparatus according to a further aspect of the present invention, said illumination optical system comprises a plurality of reflective surfaces and said reflective surface A is a final reflective surface in an optical path for guiding the light from the light source means to said display means.
In the above-stated image display apparatus according to a further aspect of the present invention, when said illumination principal rays are projected onto the xz plane of said coordinate system and when xcex8x represents an angle between a normal at a hit point of each illumination principal ray on the reflective surface A and a line of a projection of said each illumination principal ray onto the xz plane of said coordinate system, each illumination principal ray is incident to said reflective surface A so as to satisfy the following condition:
|xcex8x|xe2x89xa65xc2x0. 
In the above-stated image display apparatus according to a further aspect of the present invention, when the illumination principal rays are projected onto the yz plane of said coordinate system, when xcex8y represents an angle between a normal at a hit point of each illumination principal ray on the reflective surface A and a line of a projection of said each illumination principal ray onto the yz plane of said coordinate system, and when n represents a refractive index of a medium before and after reflection on the reflective surface A, each illumination principal ray is incident to said reflective surface A so as to satisfy the following condition:
arcsin(1/n)xe2x89xa6xcex8yxe2x89xa61.5*arcsin(1/n). 
An image display apparatus according to another aspect of the present invention is an image display apparatus for permitting an observer to observe image information displayed on display means, comprising: reflective display means; an illumination optical system having a reflective surface A for guiding light from light source means to the display means; and a display optical system for guiding light from the display means to an observation position of the observer, wherein when principal rays are defined as rays emerging from a display surface of the display means and impinging upon the center of an exit pupil, when a center-view-angle principal ray is defined as a ray emerging from the center of the display surface of the display means and passing the center of the exit pupil of the display optical system, when illumination principal rays are defined as rays emerging from the light source means and guided onto the display surface by the illumination optical system to become the principal rays, an angle of incidence of each illumination principal ray onto the display surface and an angle of emergence of a corresponding principal ray from the display surface make a predetermined angle, and wherein when a center illumination principal ray is defined as a ray emerging from the light source means and guided to the center of the display surface by the illumination optical system to become the center-view-angle principal ray, when a coordinate system is so defined that a yz plane is set on a surface formed by incidence and emergence of the center illumination principal ray to and from the reflective surface A and that a z-axis is taken along a direction of a normal to the reflective surface A in the yz plane, a y-axis along a tangent direction thereto, and an x-axis along a direction normal to the yz plane, and when the illumination principal rays are projected onto the xz plane of the coordinate system, the illumination principal rays projected onto the xz plane are incident at respective angles of incidence approximately equal to each other to the reflective surface A.
In the above-stated image display apparatus according to a further aspect of the present invention, said reflective surface A acts to totally reflect the light when the light from said light source means is guided to said display means and said reflective surface A acts to transmit the light when the light from the display means is guided to the observation position.
In the above-stated image display apparatus according to a further aspect of the present invention, when the illumination principal rays are projected onto the xz plane of the coordinate system and when xcex8x represents an angle between a normal at a hit point of each illumination principal ray on the reflective surface A and a line of a projection of the each illumination principal ray onto the xz plane of the coordinate system, each illumination principal ray is incident to the reflective surface A so as to satisfy the following condition:
|xcex8x|xe2x89xa65xc2x0. 
In the above-stated image display apparatus according to a further aspect of the present invention, when the illumination principal rays are projected onto the yz plane of the coordinate system, when xcex8y represents an angle between a normal at a hit point of each illumination principal ray on the reflective surface A and a line of a projection of the each illumination principal ray onto the yz plane of the coordinate system, and when n represents a refractive index of a medium before and after reflection on the reflective surface A, each illumination principal ray is incident to the reflective surface A so as to satisfy the following condition:
arcsin(1/n)xe2x89xa6xcex8yxe2x89xa61.5*arcsin(1/n). 
An image display apparatus according to another aspect of the present invention is an image display apparatus comprising: reflective display means; an illumination optical system having a reflective surface A; and a display optical system for guiding light from the display means to an observation position, said apparatus comprising means whereby when a yz plane is defined as a plane formed by incidence and emergence of a center illumination principal ray to and from the reflective surface A, said center illumination principal ray being defined as a ray guided to the center of a display surface of the display means by the illumination optical system and emerging from the center of the display surface of the display means to pass the center of the exit pupil of the display optical system, and when a coordinate system is so set that a z-axis is taken along a direction of a normal to the reflective surface A in the yz plane, a y-axis along a tangent direction thereto, and an x-axis along a direction normal to the yz plane, illumination principal rays guided onto the display surface of the display means by the illumination optical system and emerging from the display surface of the display means to pass the center of the exit pupil of the display optical system are made incident to and emergent from the display surface of said display means each at a predetermined angle and angles of incidence to the reflective surface A on a projection of the respective illumination principal rays onto the xz plane are controlled so as to be approximately constant.